It has been failing on Windows (bug 127480).
Marked as flaky in https://trac.webkit.org/r167531.
Could you please post a diff? Looking at flakiness dashboard, I don't see this test having ever failed.
It started flaking with my patch in https://trac.webkit.org/r167530. Not really sure why. My change is unrelated to the tested code.
Could be memory corruption from
I wanted to conjecture memory corruption from a previous test, but that's not accurate.
Turns out that custom attributes on attribute nodes are broken in JSC. We used to know that and to expect a failure, until Filip's patch <http://trac.webkit.org/changeset/159705> slightly changed memory layout, so the test started to pass accidentally. He then removed the failing expectation.
But there was no real fix, and now your patch slightly changed memory layout again, making the test fail again.
*** This bug has been marked as a duplicate of bug 88045 ***